1. Field of the Invention
The present invention relates to an AM radio receiver adapted for receiving amplitude-modulated signals, and more particularly it pertains to an AM radio receiver of the so-called up-conversion type, which comprises a radio-frequency tuning circuit for selecting amplitude-modulated signal received at antenna; a local oscillator circuit for generating an oscillating signal which is used in a first frequency conversion section to convert the receiving signal to an intermediate frequency signal having a higher frequency than that of the receiving signal.
2. Description of the Prior Art
Referring to FIG. 7 of the accompanying drawings, there is shown a circuit diagram of prior-art AM radio receiver, which includes a radio-frequency tuning circuit 71 arranged to select an amplitude-modulated signal received at an antenna, and a local oscillator circuit 72 arranged to generate an oscillating signal for converting the receiving signal to an intermediate frequency signal having a higher frequency than that of the receiving signal.
The radio-frequency tuning circuit 71 comprises a tuning coil L9, and variable capacitance diodes D14 and D15 having their cathodes connected together. The pair of variable capacitance diodes D14 and D15 are connected in parallel with the tuning coil L9.
The local oscillator circuit 72 comprises an oscillator coil L10, a capacitor C9, and a variable capacitance diode D16. The capacitor C9 is connected in parallel with the oscillator coil L10, and the variable capacitance diode D26 is connected in series therewith.
The radio-frequency tuning circuit 71 is connected to a D/A converter circuit 73 at the cathodes of the variable capacitance diodes D14 and D15 through a resistor R19. The D/A circuit 73 is connected to a microcomputer 74. Further, the local oscillator circuit 72 is connected to a microcomputer 74 at the cathode of the variable capacitance diode D16 through a resistor R20. The microcomputer 74 is coupled to an EPROM 75.
The radio-frequency tuning circuit 71 has the circuit constants thereof set up such that amplitude-modulated signals in the range from 522 KHz to 1710 KHz can be tuned in. The local oscillator circuit 72 has the circuit constants thereof set up such that oscillating frequency can be varied in the range from 11222 KHz to 12410 KHz, for the purpose of setting the first intermediate frequency signal to 10.7 MHz at the output of first frequency conversion section. To this end, the variable capacitance diodes of the radio-frequency tuning circuit 71, and the variable capacitance diode of the local oscillator circuit 72 are differentiated from each other in terms of capacitance value and variable capacitance ratio as well as characteristic curve for voltage-to-capacitance value.
With the above-described conventional AM radio receiver, because of the radio-frequency tuning circuit and local oscillator circuit using variable capacitance diodes having different characteristics, the frequency difference between the oscillating frequency of the local oscillator circuit 72 and the tuning frequency of the radio-frequency tuning circuit 71 tends to be deviated far from the desired first intermediate frequency (10.7 MHz), thus resulting in an increased tracking error, when common tuning voltage is applied to the variable capacitance diodes of the radio-frequency tuning circuit 71 and the variable capacitance diode of the local oscillator circuit 72.
Disadvantageously, therefore, the conventional AM radio receiver of the above-mentioned type using variable capacitance diodes having different characteristics for the radio-frequency tuning circuit and local oscillator circuit, requires corrective means for establishing a proper variable capacitance ratio required in the respective circuits and for making approximate the slopes of the different characteristic curves of the variable capacitance diodes employed with the respective circuits, with a view to restraining occurrence of tracking error.
In the conventional AM radio receiver illustrated in FIG. 7, such corrective means is constituted by the D/A converter circuit 73, microcomputer 74 and EPROM 75. More specifically, the arrangement is made such that a first tuning voltage provided by the microcomputer 74 is applied to the local oscillator circuit 72 and a second tuning voltage which is provided by the microcomputer 74 on the basis of prestored data in the EPROM 75 is applied to the radio-frequency tuning circuit 71 via the D/A converter circuit 73 so that variable voltage source for the local oscillator circuit 72 and that for the radio-frequency tuning circuit 71 are separately provided, applying different voltages to radio-frequency tuning circuit 71 and local oscillator circuit 72 respectively, thereby reducing tracking error. As will be seen, the above-mentioned conventional AM radio receiver is disadvantageous in that it requires such components as the D/A converter circuit, microcomputer and EPROM and thus the cost of manufacture thereof is increased accordingly.